Sulfuric acid electrolysis method and sulfuric acid electrolysis apparatus

ABSTRACT

An electrolysis apparatus comprising: an electrolytic cell in which a sulfuric acid solution is fed and discharged; a conductive anode and cathode electrode of diamond composition; a feeding unit for feeding the sulfuric acid solution to the electrolytic cell; a power supply unit for applying a voltage between the anode and cathode electrodes; and a power control unit for controlling the power supply unit such that a forward voltage is applied between the anode and cathode during normal electrolysis with the polarity applied between the anode and cathode inverted under predetermined conditions during intervals between normal operation to dissolve precipitates of sulfur generated in the electrolytic cell for stabilizing the electrolysis operation.

TECHNICAL FILED

The present invention relates to an electrolysis method and anelectrolysis apparatus of electrolyzing sulfuric acid to generatepersulfuric acid.

BACKGROUND ART

Methods of electrolyzing sulfuric acid solution to generateperoxydisulfuric acid and peroxymonosulfuric acid (hereinafter,collectively referred to as persulfuric acid) and using the persulfuricacid for cleaning a semiconductor material are known.

In one of the methods of electrolyzing sulfuric acid to generatepersulfuric acid, while passing sulfuric acid solution betweenelectrodes in an electrolytic cell, electrolysis is performed byapplying a DC voltage between an anode and a cathode of the electrodes.As structures of the electrolytic cells, there are a single-polaritycell (a pair of anode and cathode is used) and a plural-polarity cellusing a bipolar electrode. In these structures of the electrolyticcells, a relationship between the anode and cathode as one pair is thesame. A spacer is usually used in order to maintain a constant distancebetween the electrodes, and a sealing member such as an O-ring is usedin order to seal the electrolyte. For example, the inventor of theinvention proposed an electrolytic cell having the above-describedstructure (refer to Patent Document 1).

FIG. 7A schematically illustrates an electrolytic cell. A spacer 22 isdisposed between an anode 20 and a cathode 21 to secure a passage 23. Inthe spacer 22, an inlet hole 22 a is formed at inlet side of the spacer22 and an outlet hole 22 b is formed at outlet side of the spacer 22.The inlet hole 22 a and the outlet hole 22 b are configured as passageswhich are relatively narrower than the passage 23. In addition, O-rings24 as sealing members are disposed between the spacer 22 and theelectrodes to ensure sealing of the passage 23.

CITATION LIST Patent Document

Patent Document 1: Japanese Unexamined Patent Application PublicationNo. 2007-262531

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

As generally known, if sulfuric acid is electrolyzed, H₂ (gas) isgenerated at the cathode, and if sulfuric acid is exposed to a reducingatmosphere, S (solid sulfur) or H₂S (gas) is generated.

Therefore, if operation is continuously performed, sulfur or chemicalspecies associated with generation of the sulfur is generated on theelectrode surface, particularly, in a peripheral end portion of theelectrode or in a shadow portion of an O-ring. Fine S particles that areattached and grown on the electrode surface are peeled off from theelectrode and moved along the flow of the electrolyte to be attached andaccumulated in narrow portions of an outlet hole of the electrolyticcell or an inlet hole of the electrolytic cell, so that there is aproblem in that the inlet hole or the outlet hole is clogged in duecourse. It is understood that the problem easily occurs in the casewhere a concentration of sulfuric acid is high, the case where a currentdensity is high, and the case where a voltage between the electrodes ishigh.

A state where sulfur is accumulated in the O-ring portion or in thevicinity of the outlet hole is illustrated in FIG. 7B. Presumptively, inthe figure, sulfur is precipitated as indicated by “A”, and theprecipitate is peeled off and moved finally, so that the cell outlet orthe cell inlet is clogged as indicated by “B” in due course.

The invention is to provide an electrolysis method and an electrolysisapparatus capable of preventing accumulation of sulfur precipitatedthrough electrolysis of sulfuric acid solution and preventing theprecipitates of sulfur from clogging inner portions of the system.

Means For Solving Problem

According to a first aspect of the invention, there is provided asulfuric acid electrolysis method of generating persulfuric acid byperforming electrolysis in an electrolytic cell while flowing sulfuricacid solution of 70 mass % or more between an anode and an cathode amonga plurality of electrodes which include at least the anode and cathode,each of which at least liquid contact surface is constructed with aconductive diamond, the method including: performing a normal operationof performing the electrolysis by applying a forward voltage between theanode and the cathode of the electrodes; performing a polarity inversionoperation of inverting the voltage applied between the anode and thecathode in an interval between the normal operations; and dissolvingprecipitates of sulfur generated in the electrolytic cell during thenormal operation into the sulfuric acid solution during the polarityinversion operation.

According to a second aspect of the invention, in the sulfuric acidelectrolysis method according to the first aspect of the invention, theelectrolysis is performed while introducing the sulfuric acid solutionfrom an outside of the electrolytic cell into an inside of theelectrolytic cell, and the electrolyzed sulfuric acid solution isdischarged to the outside of the electrolytic cell.

According to a third aspect of the invention, in the sulfuric acidelectrolysis method according to the first or second aspect of theinvention, a retention portion where the flowing sulfuric acid solutionis retained is included in the electrolytic cell.

According to a fourth aspect of the invention, in the sulfuric acidelectrolysis method according to any one of the first to third aspectsof the invention, a narrow passage portion where the sulfuric acidsolution flows is included in the electrolytic cell.

According to a fifth aspect of the invention, in the sulfuric acidelectrolysis method according to the third aspect of the invention, aspacer of securing a passage of the sulfuric acid solution is disposedbetween the electrodes, and the retention portion is formed with thespacer or with the spacer and other members.

According to a sixth aspect of the invention, in the sulfuric acidelectrolysis method according to the fifth aspect of the invention, asealing member is installed between the spacer and the electrode, andthe retention portion is formed with at least the sealing member.

According to a seventh aspect of the invention, in the sulfuric acidelectrolysis method according to the fifth or sixth aspect of theinvention, an outlet hole as a narrow passage portion through which thesulfuric acid solution passes is formed in the spacer.

According to an eighth aspect of the invention, in the sulfuric acidelectrolysis method according to any one of the fifth to seventh aspectsof the invention, an inlet hole as a narrow passage portion throughwhich the sulfuric acid solution passes is formed in the spacer.

According to a ninth aspect of the invention, in the sulfuric acidelectrolysis method according to any one of the first to eighth aspectsof the invention, a circulation line is installed to connect an outletof the electrolytic cell and an inlet of the electrolytic cell, and anarrow passage portion is included in the circulation line and/or anupstream side of a retention portion of the electrolytic cell.

According to a tenth aspect of the invention, in the sulfuric acidelectrolysis method according to any one of the first to ninth aspectsof the invention, the polarity inversion operation is performed aftercontinuously performing the normal operation for a predetermined time.

According to an eleventh aspect of the invention, in the sulfuric acidelectrolysis method according to any one of the first to tenth aspectsof the invention, the polarity inversion operation is performed based ona result of determination of a precipitated state of the sulfur.

According to a twelfth aspect of the invention, in the sulfuric acidelectrolysis method according to any one of the first to eleventhaspects of the invention, at least one of the following conditions (a)to (c) is satisfied:

(a) a concentration of sulfuric acid in the electrolytic cell is 85 mass% or more;

(b) a temperature of sulfuric acid at the inlet of the electrolytic cellis 70° C. or more; and (c) a current density in the electrolysis is 50A/dm² or more.

According to a thirteenth aspect of the invention, there is provided asulfuric acid electrolysis apparatus including:

an electrolytic cell which sulfuric acid solution can be fed to anddischarged from;

a plurality of electrodes including at least an anode and a cathode inthe electrolytic cell, which are disposed with a gap so that thesulfuric acid solution flows between the anode and cathode and each ofwhich at least liquid contact surface is constructed with a conductivediamond;

a spacer which secures the gap of the electrodes;

a narrow passage portion formed in the spacer where the sulfuric acidsolution flows;

a feeding unit which feeds the sulfuric acid solution to theelectrolytic cell;

a power supply unit which applies a voltage between the anode and thecathode of the electrodes; and

a power control unit which controls the power supply unit to apply aforward voltage between the anode and the cathode during normalelectrolysis and to perform polarity inversion of inverting the voltageapplied between the anode and the cathode in a predefined condition.

According to a fourteenth aspect of the invention, there is provided asulfuric acid electrolysis apparatus including:

an electrolytic cell which sulfuric acid solution can be fed to anddischarged from;

a plurality of electrodes including at least an anode and a cathode inthe electrolytic cell, which are disposed with a gap so that thesulfuric acid solution flows between the anode and cathode and each ofwhich at least liquid contact surface is constructed with a conductivediamond;

a spacer which secures the gap of the electrodes;

a retention portion formed with the spacer or with the spacer and othermembers, where the sulfuric acid solution is retained;

a power supply unit which applies a voltage between the anode and thecathode of the electrodes; and

a power control unit which controls the power supply unit to performpolarity inversion of inverting a voltage applied between the anode andthe cathode during normal electrolysis.

According to the invention, before the solid sulfur or precursorsthereof are accumulated on the electrode surface or the shadow portionof the O-ring by allowing the sulfuric acid solution to flow between theelectrodes while applying a voltage between the electrodes, the polarityinversion is performed to allow the electrode surface to have anoxidizing property, and the electrolysis is performed for apredetermined time or more, so that the solid sulfur or precursorsthereof can be effectively returned to the sulfuric acid or the sulfateions. The polarity inversion operation can be continuously performed forabout 10 to 100 hours.

The above operation is repeated at a predetermined interval, so that theaccumulation of sulfur and the clogging of the cell can be prevented. Asa method of determining the interval, the interval can be determinedaccording to a certain operation time based on experience or the numberof electronic material boards processed by cleaning or the like.However, if sulfur is accumulated, a voltage (voltage between theelectrodes) required to flow a predetermined current is increased.Therefore, when the conduction is formed through current control, thevoltage can be always monitored. When the voltage is increased up to apredetermined value, the polarity inversion operation may be configuredto be started. In addition, even when the polarity inversion isperformed, the voltage can be monitored. When the voltage is increaseddown to a predetermined value, the polarity inversion operation may beconfigured to be stopped.

Hereinafter, forms of reaction during the electrolysis are described.

In the electrolyte, the sulfuric acid or water molecules are dissociatedas follows, so that ions of SO₄ ²⁻, HSO₄ ⁻, H⁺, and the like exist.

H₂SO₄

HSO₄ ⁻+H⁺

HSO₄ ⁻

SO₄ ²⁻+H⁺

H₂O

OH⁻+H⁺

The concentrations of H⁺ (same as H₃O⁺) and HSO₄ ⁻ have peaks at theconcentration of sulfuric acid of 70 mass % to 80 mass % and aredecreased at the higher concentration side. On the other hand, theconcentration of undissociated sulfuric acid molecules H₂SO₄ (aq) isdrastically increased at the higher concentration side. In addition,since a high concentration sulfuric acid solution is a strong acid, theconcentration of OH⁻ is low.

At the cathode, H⁺ ions are attracted, and thus, the H⁺ ions receiveelectrons as expressed in the following reaction formula, so that thehydrogen gas H₂ is generated.

2H⁺+2e⁻→H₂

At the anode, HSO₄ ⁻ or SO₄ ²⁻ ions are attracted, and thus, the HSO₄ ⁻or SO₄ ²⁻ ions release electrons as expressed in the following reactionformula, so that the persulfuric acid H₂S₂O₃ is generated.

2HSO₄ ⁻→S₂O₈ ²⁻+2H⁺+2e⁻

2SO₄ ²⁻→S₂O₈ ²⁻+2e⁻

In addition, at the anode, water is also electrolyzed as expressed inthe following reaction formula, so that oxygen gas O₂ is generated.

2OH⁻→O₂+2H⁺+4e⁻

With respect to oxidization and reduction potentials in the electrodereaction related to the sulfuric acid and the water, Pourbaix diagramsillustrated in FIGS. 4 and 5 are known.

If the potentials of the sulfuric acid and the water under theconditions of the concentration of sulfuric acid=92 mass % and thetemperature=60° C. are illustrated based on the Pourbaix diagram andjuxtaposed, the potentials are as shown in FIG. 6. In the figure, thenumbers of the formulas are based on the formulas on the diagramsillustrated in FIGS. 4 and 5.

Since the sulfuric acid used in the electrolytic cell has a highconcentration, pH is almost −2. At the anode side, O₂, O₃, H₂O₂, andH₂S₂O₈ are generated with the potential order of O₂>O₃>H₂O₂>H₂S₂O₈.Actually, the largest amount of O₂ is generated, and the second largestamount of H₂S₂O₈ is generated. The potential order at the cathode isS>H₂>H₂S. In an actual cell, although the mainly generated material isH₂, S can be sufficiently obtained in terms of potential. If thegenerated S is not retained on the electrode surface or the like, thegenerated S reacts with oxidizing substance in the solution to return tosulfuric acid. If there is a retention portion, S is accumulated on theretention portion.

With respect to efficiency of the generation of persulfuric acid,actually, 80 to 90% of electrons passing through the anode areassociated with the generation of O₂, and remaining 10 to 20% iscontributed to the generation of persulfuric acid. The O₂ is dischargedto the outside of the system, and the persulfuric acid is used foroxidation reaction to become sulfuric acid so as to return to the cell.If the solution is used in the above-described circulation manner, wateris consumed in the electrolytic cell where the sulfuric acid iselectrolyzed, so that the concentration of sulfuric acid is increased.

As described above, although it is difficult in principle that thegeneration of sulfur at the cathode is completely prevented, the sulfuris allowed to return to the sulfuric acid by oxidizing the sulfur byusing the oxidizing substance (persulfuric acid or the like) generatedat the anode. In the case where the sulfur is not well oxidized due tothe retention of the solution, it is preferable that the polarityinversion be performed to generate persulfuric acid in the vicinity ofthe retention portion so that the sulfur in the retention portion isremoved.

The invention is appropriate for the case where a retention portion inwhich sulfuric acid solution is retained is formed in the electrolyticcell. The retention portion does not denote a specific position in theelectrolytic cell, and the retention portion may be formed at differentpositions according to the structure of the electrolytic cell. Theretention portion is easily formed at the position where the flow of thesulfuric acid solution is disturbed.

The retention portion is easily formed in a concave portion such as acorner of a member where the angle of the surface is rapidly changed.The retention portion may be formed in an intersection portion betweenthe electrode and the spacer and an intersection portion between thesealing member and the electrode or the spacer.

In the case where the electrolytic cell has a narrow passage portion ofwhich passage cross section is relatively smaller than other portions,since the passage is easily clogged by precipitates, the invention isparticularly useful for the electrolytic cell having the narrow passageportion. In the case where the narrow passage portion is disposed at thedownstream side of the electrolytic cell, the problem easily occurs, forexample, at the outlet hole or like. In addition, the precipitatesaccumulated in the retention portion are peeled off and flowed togetherwith the sulfuric acid solution, so that there may be a problem in thatthe precipitates are attached and accumulated in the inlet hole of theelectrolytic cell to prevent the flow of the sulfuric acid solution byclogging the inlet hole in due course.

According to results of previous research of the inventors, in the casewhere the concentration of sulfuric acid is 85 mass % or more and thecurrent density is 50 A/dm² or more, sulfur can be easily precipitatedand accumulated. In the case where the concentration of sulfuric acid ishigher than the above value, the current density needs to be set to belower than the above value. In addition, the operation is performed sothat the temperature of the inlet of the cell is in a range of 40 to 70°C. However, it may be understood that, if conduction resistance isincreased due to the precipitation of sulfur, inner resistance of thecell is increased, so that the temperature of the cell is increased.Therefore, the precipitation of sulfur is accelerated. Quantitatively,it may be understood that this is because vaporization of water isaccelerated and, thus, the concentration is remarkably increased,particularly, at the retention portion or the like illustrated in FIG.7. Particularly, it is not preferable that the operation be performed atthe temperature of the inlet of the cell exceeding 70° C.

It can be understood from the above description that the problem ofprecipitates of sulfur easily occurs when any one of the following threeconditions is satisfied. Therefore, it is preferable that the inventionbe performed in the case where a system is under the followingconditions.

(a) The concentration of sulfuric acid in the electrolytic cell is 85mass % or more.

(b) The temperature of sulfuric acid at the inlet of the electrolyticcell is 70° C. or more.

(c) The current density is 50 A/dm² or more.

The concentration of TOC after cleaning in an electronic materialcleaning by using sulfuric acid solution is in a range of 0 to 10 mg/l,and organic materials do not almost exist.

Effect of the Invention

As described above, according to the invention, when electrolyzingsulfuric acid solution in an electrolytic cell, the problem ofaccumulation of precipitates of sulfur or precursors thereof is avoided,so that it is possible to obtain an effect in that stable electrolysiscan be continuously performed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a single-wafer-type cleaning systemincluding an electrolytic cell according to an embodiment of theinvention.

FIG. 2 is a diagram illustrating a batch-type cleaning system includingan electrolytic cell according to an embodiment of the invention.

FIG. 3 is a diagram illustrating a batch-type cleaning system using anelectrolytic cell without a polarity inversion function.

FIG. 4 is a diagram illustrating a Pourbaix diagram.

FIG. 5 is a diagram illustrating a Pourbaix diagram.

FIG. 6 is a graph illustrating potential of sulfuric acid having aconcentration of 92 mass % and a temperature of 60° C. and potential ofwater based on Pourbaix diagram.

FIG. 7 is a diagram illustrating a structure of an electrolytic cell andan accumulated state of sulfur.

MODE FOR CARRYING OUT THE INVENTION

(First Embodiment)

Hereinafter, a cleaning system including an electrolysis apparatusaccording to an embodiment of the invention will be described withreference to the attached drawings.

As illustrated in FIG. 1, an electrolysis apparatus 1 is configured toinclude an electrolytic cell 2. The electrolytic cell 2 is of anon-diaphragm type, and an anode and a cathode which are configured withdiamond electrodes are installed within the electrolytic cell 2 withoutseparation by a diaphragm. As a power supply unit, a DC power supplyunit 3 is connected to the two electrodes. A power control unit 4 whichcontrols the direction of voltage applied to the anode and the cathodeis connected to the DC power supply unit 3. The power control unit 4 maybe configured to include a switching element which switches a path ofthe voltage applied from the DC power supply unit 3 to the anode and thecathode, for example.

As illustrated in FIG. 7A, the electrolytic cell 2 is configured toinclude an anode 20 and a cathode 21 which are configured withplate-shaped diamond electrodes. A spacer 22 is disposed between theanode 20 and the cathode 21 to secure a passage 23 between the anode 20and cathode 21. As the diamond electrode, a diamond electrode having aconductivity which is obtained by forming a diamond thin film in asubstrate shape and doping boron in a range of, preferably, 50 to 20,000ppm with respect to carbon amount of the diamond thin film may beappropriately used.

In the description of the embodiment, the electrolytic cell isconfigured to include an anode and a cathode as the electrodes. However,besides the anode and the cathode as the electrodes, the electrolyticcell can be configured to include a bipolar electrode. In addition, inthe electrolytic cell, multiple layers of electrodes are installed sothat gaps are formed between the electrodes, and electrolysis isperformed by allowing the sulfuric acid solution to pass between theelectrodes.

In the spacer 22, an inlet hole 22 a is formed at inlet side of thespacer 22 and an outlet hole 22 b is formed at outlet side of the spacer22. The inlet hole 22 a and the outlet hole 22 b are configured aspassages which are relatively narrower than the passage 23. The inlethole 22 a and the outlet hole 22 b correspond to narrow passage portionsin the invention. In addition, O-rings 24 as sealing members aredisposed between the spacer 22 and the anode 20 and between the spacer22 and the cathode 21 to ensure sealing of the passage 23. As a materialconstituting the spacer 22, a material (for example,polytetrafluoroethylene) having an insulating property and corrosionresistance is preferred.

In the electrolytic cell 2, the inner surfaces of the electrodes 20 and21, the corner portions of the spacer 22, or the inner surface side ofthe O-ring 24 become retention portions 25 which disturb the flow of thesulfuric acid solution. In order to form an upward flow, theelectrolytic cell 2 is arranged so that the inlet side becomes the lowerside and the outlet side becomes the upper side.

An electrolyte reservoir 10 is connected to the electrolytic cell 2through a first circulation line 5 so that the sulfuric acid solutioncan be circulated and flowed between the electrolytic cell 2 and theelectrolyte reservoir 10. Namely, the feeding side of the firstcirculation line 5 is connected to the electrolytic cell 2 so as tocommunicate with the inlet side of the electrolytic cell 2, and thereturning side of the first circulation line 5 is connected to theelectrolytic cell 2 so as to communicate with the outlet side of theelectrolytic cell 2.

A gas liquid separation tank 6 is installed at the returning side of thefirst circulation line 5. The gas liquid separation tank 6 receivessulfuric acid solution containing gas and separates the gas from thesulfuric acid solution to discharge the gas to the outside of thesystem. As the gas liquid separation tank 6, any known component may beused. In the invention, if separation of gas from solution is available,the configuration is not particularly limited.

In addition, a circulation pump 7 which circulates the sulfuric acidsolution and a cooler 8 which cools the sulfuric acid solution areinstalled at the feeding side of the first circulation line 5. The firstcirculation line 5 and the circulation pump 7 correspond to a feedingunit in the invention. The cooler 8 cools the sulfuric acid solution toadjust the temperature thereof to be appropriate for the electrolysis,for example, a temperature in a range of 40 to 70° C. The invention isnot particularly limited to the above-described configuration. Theelectrolysis apparatus according to the invention is configured with theelectrolytic cell 2, the DC power supply unit 3, the power control unit4, the first circulation line 5, the gas liquid separation tank 6, thecirculation pump 7, and the cooler 8.

The feeding side of a second circulation line 11 is connected to theelectrolyte reservoir 10 through a feeding pump 12.

A heater 13 is installed in the feeding direction of the secondcirculation line 11. The front end side of the second circulation line11 in the feeding direction at the downstream side of the heater 13 isconnected to the single-wafer-type cleaning apparatus 15.

The heater 13 is configured to include a quartz pipe to heat thesulfuric acid solution in a one pass manner, for example, by a nearinfrared heater. As a result, the sulfuric acid solution is rapidlyheated so that the temperature of the sulfuric acid solution in thesingle-wafer-type cleaning apparatus 15 is in a range of 150 to 220° C.

In the above-described single-wafer-type cleaning apparatus 15, forexample, a electronic material substrate 100 is fixed to be mounted on arotation table, and a process of allowing the sulfuric acid solutioncontaining persulfuric acid to flow down from a nozzle to thesemiconductor material is performed.

In the description of the embodiment, the cleaning apparatus is of asingle-wafer-type cleaning apparatus. However, the type of cleaningapparatus according to the invention is not limited to the above type,but a batch-type cleaning apparatus may be used.

The returning side of the second circulation line 11 is connected to thesingle-wafer-type cleaning apparatus 15. At the returning side of thesecond circulation line 11, along the returning direction, a pump 16, areaction tank 17, a feeding pump 18, and a cooler 19 are sequentiallyinstalled, and the front end side of the second circulation line 11 inthe returning direction is connected to the electrolyte reservoir 10.

Next, operations of the cleaning system having the above-describedconfiguration will be described.

Sulfuric acid solution having a sulfuric acid concentration of 85 to 96mass % and a temperature of 50 to 80° C. is stored in the electrolytereservoir 10. The sulfuric acid solution is fed to the first circulationline 5 by the circulation pump 7. The cooler 8 adjusts the temperatureof the sulfuric acid solution to be appropriate for the electrolysis (40to 70° C.), and the sulfuric acid solution is introduced to the inletside of the electrolytic cell 2 to be flowed from the inlet hole 22 ainto the passage 23.

In the electrolytic cell 2, a forward voltage is applied between theanode and the cathode by the DC power supply unit 3, so that thesulfuric acid solution introduced into the electrolytic cell 2 iselectrolyzed. Due to the electrolysis, in the electrolytic cell 2, anoxidizing substance including the persulfuric acid and oxygen gas aregenerated at the anode side, and hydrogen gas is generated at thecathode side. The oxidizing substance and the gases are flowed throughthe passage 23 in the state where the oxidizing substance and the gasesare mixed with the sulfuric acid solution. The sulfuric acid solutionflowing through the passage 23 is fed to the first circulation line 5through the outlet hole 22 b. The sulfuric acid solution discharged fromthe outlet hole 22 b is fed to the gas liquid separation tank 6 throughthe first circulation line 5, so that the gases are separated from thesulfuric acid solution. The gases are discharged to the outside of thesystem to be processed safely by a catalytic apparatus (not illustrated)or the like.

The sulfuric acid solution from which gas is separated by the gas liquidseparation tank 6 contains persulfuric acid, and the sulfuric acidsolution is allowed to return to the electrolyte reservoir 10 throughthe returning side of the first circulation line 5. After that, thesulfuric acid solution is repetitively fed to the electrolytic cell 2,and thus, the concentration of persulfuric acid can be increased byelectrolysis. If the concentration of persulfuric acid reaches anappropriate concentration, a portion of the sulfuric acid solution inthe electrolyte reservoir 10 is fed to the heater 13 through the feedingside of the second circulation line 11 by the feeding pump 12.

In the heater 13, while passing through the passage, the sulfuric acidsolution containing persulfuric acid is heated by a near infraredheater. It is preferable that, during the feeding of the sulfuric acidsolution, the flow rate of the sulfuric acid solution be adjusted suchthat the passing time from the entrance of the heater 13 until thesulfuric acid solution is used in the single-wafer-type cleaningapparatus 15 is less than 1 minute, preferably, less than 20 seconds,more preferably, less than 10 seconds. In the single-wafer-type cleaningapparatus 15, a flow rate of 500 to 2000 mL/minute is considered to bean appropriate amount, and thus, a passage length and passagecross-section area of the heater 13 and a line length and a passagecross-section area of the second circulation line 11 at the downstreamside of the heater 13 and the like are set so that the above-describedpassing time is less than 1 minute at the flow rate. In thesingle-wafer-type cleaning apparatus 15, when the sulfuric acid solutionis supplied to the electronic material substrate 100, the temperature ofthe liquid is in a range of 150° C. to 220° C.

A cleaning object of the single-wafer-type cleaning apparatus 15 is asemiconductor material such a silicon wafer where resist implanted withions of, for example, 1×10¹² to 1×10¹⁶ atoms/cm² is formed.

Contaminants such as resist on the electronic material substrate 100 iseffectively peeled off and removed by flowing and dropping a smallamount of the high-temperature sulfuric acid solution containingpersulfuric acid from a nozzle (not illustrated) to be in contact withthe electronic material substrate 100 while rotating the electronicmaterial substrate 100 on a rotation table (not illustrated).

The sulfuric acid solution used for the cleaning is discharged from thesingle-wafer-type cleaning apparatus 15 and is fed to the reaction tank17 through the returning side of the second circulation line 11 by thepump 16 to be stored in the reaction tank 17. The sulfuric acid solutionstored in the reaction tank 17 contains residual organic materials suchas resist cleaned by the single-wafer-type cleaning apparatus 15, andduring the storage of the sulfuric acid solution in the reaction tank17, the residual organic materials are oxidatively decomposed byoxidizing substance contained in the sulfuric acid solution. The storagetime of the sulfuric acid solution in the reaction tank 17 can bearbitrarily adjusted according to residual organic material content orthe like. At this time, since the high-temperature sulfuric acidsolution containing persulfuric acid is continuously supplied from thesingle-wafer-type cleaning apparatus 15, the reaction tank 17 ismaintained at an appropriate temperature.

In the reaction tank 17, the sulfuric acid solution where the containedresidual organic material is oxidatively decomposed is circulated to theelectrolyte reservoir 10 through the cooler 19 installed in the secondcirculation line 11 by the feeding pump 18.

In addition, if the high-temperature sulfuric acid solution iscirculated to the electrolyte reservoir 10, the decomposition of thepersulfuric acid in the sulfuric acid solution stored in the electrolytereservoir 10 is accelerated. Therefore, after the sulfuric acid solutionis cooled down to an appropriate temperature of about 50 to 80° C. bythe cooler 19, the sulfuric acid solution is introduced into theelectrolyte reservoir 10. The sulfuric acid solution introduced into theelectrolyte reservoir 10 is fed to the electrolytic cell 2 through thefeeding side of the first circulation line 5, and persulfuric acid isgenerated by electrolysis. The sulfuric acid with the persulfuric acidis fed to the electrolyte reservoir 10 through the returning side of thefirst circulation line 5. The circulation is repeated, so that thepersulfuric acid is continuously generated.

According to the above-described operation, the sulfuric acid solutioncontaining persulfuric acid is fed to be circulated, so that thehigh-temperature cleaning solution containing the high-concentrationpersulfuric acid can be continuously supplied to the single-wafer-typecleaning apparatus 15 as a using side.

Although not described above, a discharging line can be connected to thesecond circulation line 11 at the upstream side of the reaction tank 17to be branched from the second circulation line 11, so that the sulfuricacid solution is not fed to the reaction tank 17 but discharged to theoutside of the system at an appropriate time.

By discharging a small amount of the sulfuric acid solution from thedischarging line as needed, it is possible to prevent resist dopingelements or other materials which are not oxidatively decomposedaccumulated in the solution in the system from being accumulated at ahigh concentration. The above operation may be performed by controllingopening and closing of an opening/closing valve installed in thecirculation line or the discharging line.

By continuously maintaining the operation state in the cleaning system,as described above, in the electrolytic cell 2, sulfur or chemicalspecies associated with generation of the sulfur is generated in theretention portion 25 on the electrode surface, particularly, in theperipheral end portion of the electrode or in the shadow portion of theO-ring. If the generation of the sulfur or the chemical species is notprevented, as described above, the sulfur or the chemical species aregradually grown to cause the above-described peeling or the clogging ofthe narrow passage.

In the invention, in a case where the continuation time of theelectrolysis reaches a predetermined time, in a case where the number ofprocessed electronic material substrates 100 reaches a predeterminednumber, or in a case where the precipitation of the sulfur componentreaches some degrees, the polarity inversion operation of inverting thevoltage applied between the anode 20 and the cathode 21 from the DCpower supply unit 3 is performed under the control of the power controlunit 4, so that the electrolysis is performed. Therefore, theprecipitates of sulfur precipitated in the vicinity of the cathode 21 orthe like is dissolved by an oxidizing substance which is generated inthe vicinity of the cathode 21 functioning as an anode due to thepolarity inversion, so that the dissolved sulfur is moved together withthe sulfuric acid solution. By maintaining the polarity inversionoperation to some extent, the precipitates of sulfur are removed ordecreased, so that the stable electrolysis can be continuouslyperformed. After the precipitates of sulfur are removed or decreased,the operation is continuously performed. Before the precipitates ofsulfur or precursors thereof are accumulated on the polarity-invertedcathode, the polarity inversion is performed again to invert the voltageapplied between the anode 20 and the cathode 21 in the backwarddirection, so that the electrolysis is performed by applying the voltagein the forward direction.

By repeating the above-described operation, the stable electrolysis canbe continuously performed for a long time.

In general, in the polarity inversion as a measure of coping withadhesion of organic materials, the polarity inversion time is limitedsince the cathode and the anode are different from each other. However,in the embodiment, since all of the electrodes are diamond electrodes,normal operation can be performed for a long time (10 to 100 hours) inthe state in which the polarity inversion is performed.

In addition, the continuation time of the operation after the polarityinversion can be set so that the time of the normal electrolysis reachesa predetermined time. The predetermined time may be determined byconsidering an accumulated current amount and a concentration,temperature, a flow rate, or the like of sulfuric acid solution, and thepredetermined time may be obtained through experiments. In addition, ina case where the number of cleaned electronic material substrates 100 isa predetermined number, the polarity inversion operation may also beperformed.

Although the continuation time of the operation after the polarityinversion may be set not to be constant, in a case where diamond layerswith uniform thickness are laminated on the two surfaces of the diamondelectrode, it is preferable that the continuation time is set to beconstant in order to equalize abrasion of the diamond according to theoperation.

In addition, as a different timing of performing the polarity inversionoperation, a result of estimation of a degree of precipitation of sulfurin the electrolytic cell can be used. Namely, in a case where anestimated degree of precipitation of sulfur reaches a predefined degree,the polarity inversion operation is performed. The degree ofprecipitation of sulfur can be determined by an electrolysis voltagerising when the electrolysis is performed at a constant current asdescribed above. Namely, when the voltage reaches a predefinedelectrolysis voltage, the precipitation of sulfur proceeds, so that thepolarity inversion operation is performed.

(Second Embodiment)

Next, a second embodiment where the above-described electrolysisapparatus 1 is applied to a batch-type cleaning tank 30 will bedescribed with reference to FIG. 2. In the second embodiment, the samecomponents as those of the first embodiment are denoted by the samereference numerals, and the description thereof will not be made or willbe simply made.

An electrolyte reservoir 10 is connected to an electrolytic cell 2through a first circulation line 5. A gas liquid separation tank 6 isinstalled at the returning side of the first circulation line 5, and acirculation pump 7 and a cooler 8 are sequentially installed at thefeeding side of the first circulation line 5.

In the batch-type cleaning tank 30, an outlet side and an inlet side areconnected to a second circulation line 31, and a feeding pump 32 and aheater 33 are installed at the returning side of the second circulationline 31. An electronic material substrate 100 is immersed in thesulfuric acid solution in the batch-type cleaning tank 30, so thatresist or the like attached on the electronic material substrate 100 ispeeled off and cleaned. At this time, while the batch-type cleaning tank30 is controlled by a heating unit (not illustrated) such as a heater ora heat exchanger so that the temperature of the batch-type cleaning tank30 is in a range of 120˜190° C., the sulfuric acid solution iscirculated.

A returning third circulation line 35 is connected to the secondcirculation line 31 at the downstream side of the feeding pump 32 andthe upstream side of the heater 33 to be branched from the secondcirculation line 31, and a feeding end side of the returning thirdcirculation line 35 is connected to the electrolyte reservoir 10 througha cooler 37.

A feeding third circulation line 34 is connected to the electrolytereservoir 10 through a feeding pump 36. The feeding third circulationline 34 is connected to the second circulation line 31 at the downstreamside of the heater 33 to merge with the second circulation line 31.

The heater 33 may have the same configuration as that of theabove-described heater 13.

Next, operations of the cleaning system having the above-describedconfiguration will be described.

Sulfuric acid solution having a concentration of 85 to 96 mass % and atemperature of 50 to 90° C. is stored in the electrolyte reservoir 10.The sulfuric acid solution is fed to the first circulation line 5 by thecirculation pump 7. The cooler 8 adjusts the temperature of the sulfuricacid solution to be appropriate for the electrolysis (40 to 80° C.), andthe sulfuric acid solution is introduced from the inlet hole 22 a of theelectrolytic cell 2 to the passage 23.

In the electrolytic cell 2, a forward voltage is applied between theanode and the cathode by the DC power supply unit 3, so that thesulfuric acid solution introduced into the electrolytic cell 2 iselectrolyzed. The electrolyzed sulfuric acid solution is fed to thefirst circulation line 5 through the outlet hole 22 b, so that gas isseparated from the gas liquid separation tank 6.

The sulfuric acid solution from which gas is separated by the gas liquidseparation tank 6 is allowed to return to the electrolyte reservoir 10through the returning side of the first circulation line 5. After that,the sulfuric acid solution is repetitively fed to the electrolytic cell2, so that the concentration of persulfuric acid can be increased byelectrolysis. If the concentration of persulfuric acid reaches anappropriate concentration, a portion of the sulfuric acid solution inthe electrolyte reservoir 10 is fed to the second circulation line 31 atthe downstream side of the heater 33 through the feeding thirdcirculation line 34 by the feeding pump 36 to merge with the sulfuricacid solution of the second circulation line 31. The merged sulfuricacid solution is introduced into the batch-type cleaning tank 30.

In addition, the sulfuric acid solution in the batch-type cleaning tank30 is circulated through the second circulation line 31 by the feedingpump 32. At this time, the sulfuric acid solution which is heated by theheater 33 is introduced into the batch-type cleaning tank 30.

In the heater 33, while the sulfuric acid solution containingpersulfuric acid passes through the passage, the sulfuric acid solutionis heated by the heater. At this time, the sulfuric acid solution isheated so that the sulfuric acid solution is mixed with the sulfuricacid solution fed through the feeding third circulation line 34 thetemperature thereof is in a range of 120° C. to 190° C. when thesulfuric acid solution is supplied to the batch-type cleaning tank 30.

The electronic material substrate 100 is cleaned in the batch-typecleaning tank 30. While a portion of the sulfuric acid solution used forthe cleaning is circulated through the second circulation line 31, theportion of the sulfuric acid solution is heated by the heater 33 to befed to the batch-type cleaning tank 30, and the remaining portion of thesulfuric acid solution is allowed to return to the electrolyte reservoir10 through the returning third circulation line 35. At this time, thesulfuric acid solution is cooled by the cooler 37 down to thetemperature thereof which is appropriate for the electrolysis, forexample, in a range of 40 to 70° C.

In the electrolyte reservoir 10, the sulfuric acid solution is fed tothe electrolytic cell 2 through the first circulation line 5 by thecirculation pump 7, so that the persulfuric acid is generated and thesulfuric acid solution is returned to the electrolyte reservoir 10.

By repeating the circulation of the sulfuric acid solution, the cleaningof the electronic material substrate 100 can be performed in the statewhere the concentration of persulfuric acid is stable.

By continuously maintaining the operation state in the cleaning system,as described above, sulfur or chemical species associated withgeneration of the sulfur is generated in the retention portion 25 in theelectrolytic cell 2. In the embodiment, the polarity inversion operationof inverting the voltage applied between the anode 20 and the cathode 21from the DC power supply unit 3 is performed under the control of thepower control unit 4 at a predetermined timing, so that the electrolysisis continuously performed. Therefore, the precipitates of sulfurprecipitated in the vicinity of the cathode or the like is dissolved. Bymaintaining the polarity inversion operation to some extent, theprecipitate portion of sulfur are removed or decreased, so that thestable electrolysis can be continuously performed.

(Comparative Example)

This comparative example has the same configuration as that of thesecond embodiment except that the power control unit 4 of the secondembodiment is not included. The comparative example will be describedwith reference to FIG. 3. In the electrolytic cell 2, a forward voltageis always applied to the anode side and the cathode side by the DC powersupply unit 3, and the electrolysis of the sulfuric acid solution can beperformed.

In this comparative example, the cleaning object such as a semiconductorsubstrate can be effectively cleaned by electrolyzing the sulfuric acidsolution. However, as time elapses, the precipitates of sulfur isgenerated in the electrolytic cell, and the peeled precipitates ofsulfur clog the narrow passage portions of the electrolytic cell 2, sothat cleaning performance may be decreased or the cleaning may becomedifficult to be performed. If the peeled precipitates of sulfur reachthe narrow passage such as the inlet hole of the electrolytic cell,there occurs the problem in that the passage is clogged or the flowbecomes worse.

Hereinbefore, the invention is described based on the embodiments.However, the invention is not limited to the embodiments, but it may beappropriately changed and modified without departing from the spirit ofthe invention.

EXAMPLE Example 1

An example using the single-wafer-type cleaning system illustrated inFIG. 1 will be described. Operation was continuously performed under theconditions of concentration of sulfuric acid=92 mass %, temperature ofthe liquid at the inlet of the electrolytic cell=60° C., and currentdensity=35 A/dm². Every time when electrolysis was continuouslyperformed for 50 hours, polarity inversion was performed. After thecontinuous electrolysis for 50 hours and the polarity inversion wasrepeated continuously 10 times for 50 hours, the cell was opened, andinternal check was performed. Attachment of sulfur was not observed atall.

Example 2

An example using the batch-type cleaning system illustrated in FIG. 2will be described. Operation was continuously performed under theconditions of concentration of sulfuric acid=85 mass %, temperature ofthe liquid at the inlet of the electrolytic cell=50° C., and currentdensity=50 A/dm². 50 wafers were processed in one batch, and polarityinversion was performed every 40 batches. After the polarity inversionwas repeated eight times, the cell was opened, and internal check wasperformed. Attachment of sulfur was not observed at all.

(Comparative Example)

The batch-type cleaning system having no polarity inversion functionillustrated in FIG. 3 was used. Operation was continuously performedunder the conditions of concentration of sulfuric acid=85 mass %,temperature of the liquid at the inlet of the electrolytic cell=50° C.,and current density=50 A/dm². When 100 batches were processed, voltagestarted to be increased, and the flow rate was gradually decreased. Whenthe flow rate per cell was decreased by half, the cell was opened andchecked. Clogging of the passage in the cell outlet portion by sulfurwas observed.

Description of the Reference Numeral

-   1 electrolysis apparatus-   2 electrolytic cell-   20 anode-   21 cathode-   22 spacer-   22 a inlet hole-   22 b outlet hole-   23 passage-   3 DC power supply unit-   4 power control unit-   5 first circulation line-   7 circulation pump-   8 cooler-   10 electrolyte reservoir-   15 single-wafer-type cleaning apparatus-   30 batch-type cleaning tank

1. A sulfuric acid electrolysis method of generating persulfuric acid byperforming electrolysis in an electrolytic cell while flowing sulfuricacid solution of 70 mass % or more between an anode and an cathode amonga plurality of electrodes which include at least the anode and cathode,each of which at least liquid contact surface is constructed with aconductive diamond, the method comprising: performing a normal operationof performing the electrolysis by applying a forward voltage between theanode and the cathode of the electrodes; performing a polarity inversionoperation of inverting the voltage applied between the anode and thecathode in an interval between the normal operations; and dissolvingprecipitates of sulfur generated in the electrolytic cell during thenormal operation into the sulfuric acid solution during the polarityinversion operation.
 2. The sulfuric acid electrolysis method accordingto claim 1, wherein the electrolysis is performed while introducing thesulfuric acid solution from an outside of the electrolytic cell into aninside of the electrolytic cell, and the electrolyzed sulfuric acidsolution is discharged to the outside of the electrolytic cell.
 3. Thesulfuric acid electrolysis method according to claim 1, wherein aretention portion where the flowing sulfuric acid solution is retainedis included in the electrolytic cell.
 4. The sulfuric acid electrolysismethod according to claim 1, wherein the electrolytic cell includes anarrow passage portion through which the flow of the sulfuric acidsolution is confined.
 5. The sulfuric acid electrolysis method accordingto claim 3, wherein a spacer of securing a passage of the sulfuric acidsolution is disposed between the electrodes, and the retention portionis formed with the spacer or with the spacer and other members.
 6. Thesulfuric acid electrolysis method according to claim 5, wherein asealing member is installed between the spacer and the electrode, andthe retention portion is formed with at least the sealing member.
 7. Thesulfuric acid electrolysis method according to claim 5, wherein theelectrolytic cell has an outlet hole forming a narrow passage portion inthe spacer through which the sulfuric acid solution passes.
 8. Thesulfuric acid electrolysis method according to claim 5, wherein theelectrolytic cell has an inlet hole forming a narrow passage portion inthe spacer through which the sulfuric acid solution passes.
 9. Thesulfuric acid electrolysis method according to claim 1, wherein acirculation line is installed to connect an outlet of the electrolyticcell and an inlet of the electrolytic cell, and a narrow passage portionis included in the circulation line and/or an upstream side of aretention portion of the electrolytic cell.
 10. The sulfuric acidelectrolysis method according to claim 1, wherein the polarity inversionoperation is performed after continuously performing the normaloperation for a predetermined time.
 11. The sulfuric acid electrolysismethod according to claim 1, wherein the polarity inversion operation isperformed based on a result of determination of a precipitated state ofthe sulfur.
 12. The sulfuric acid electrolysis method according to claim1, wherein at least one of the following conditions (a) to (c) issatisfied: (a) a concentration of sulfuric acid in the electrolytic cellis 85 mass % or more; (b) a temperature of sulfuric acid at the inlet ofthe electrolytic cell is 70° C. or more; and (c) a current density inthe electrolysis is 50 A/dm2 or more.
 13. A sulfuric acid electrolysisapparatus comprising: an electrolytic cell which sulfuric acid solutioncan be fed to and discharged from; a plurality of electrodes includingat least an anode and a cathode in the electrolytic cell, which aredisposed with a gap between the anode and cathode so that the sulfuricacid solution flows in the gap and each of which at least liquid contactsurface is constructed with a conductive diamond; a spacer which securesthe gap of the electrodes; a narrow passage portion formed in the spacerwhere the sulfuric acid solution flows; a feeding unit which feeds thesulfuric acid solution to the electrolytic cell; a power supply unitwhich applies a voltage between the anode and the cathode of theelectrodes; and a power control unit which controls the power supplyunit to apply a forward voltage between the anode and the cathode duringnormal electrolysis and to perform polarity inversion of inverting thevoltage applied between the anode and the cathode in a predefinedcondition.
 14. A sulfuric acid electrolysis apparatus comprising: anelectrolytic cell which sulfuric acid solution can be fed to anddischarged from; a plurality of electrodes including at least an anodeand a cathode in the electrolytic cell, which are disposed with a gapbetween the anode and cathode so that the sulfuric acid solution flowsin the gap and each of which at least liquid contact surface isconstructed with a conductive diamond; a spacer which secures the gap ofthe electrodes; a retention portion formed with the spacer or with thespacer and other members, where the sulfuric acid solution is retained;a power supply unit which applies a voltage between the anode and thecathode of the electrodes; and a power control unit which controls thepower supply unit to perform polarity inversion of inverting a voltageapplied between the anode and the cathode during normal electrolysis.15. The sulfuric acid electrolysis method according to claim 2, whereinthe electrolytic cell includes a narrow passage portion through whichthe flow of the sulfuric acid solution is confined.
 16. The sulfuricacid electrolysis method according to claim 3, wherein the electrolyticcell includes a narrow passage portion through which the flow of thesulfuric acid solution is confined.
 17. The sulfuric acid electrolysismethod according to claim 6, wherein the electrolytic cell has an outlethole forming a narrow passage portion in the spacer through which thesulfuric acid solution passes.